Many processes in the semiconductor industry require a reliable source of process gases for a wide variety of applications. Often these gases are stored in cylinders or vessels and then delivered to the process under controlled conditions from the cylinder. The semiconductor manufacturing industry, for example, uses a number of hazardous specialty gases such as phosphine, arsine, and boron trifluoride for doping, etching, and thin-film deposition. These gases pose significant safety and environmental challenges due to their high toxicity and pyrophoricity (spontaneous flammability in air). In addition to the toxicity factor, many of these gases are compressed and liquefied for storage in cylinders under high pressure. Storage of toxic gases under high pressure in metal cylinders is often unacceptable because of the possibility of developing a leak or catastrophic rupture of the cylinder.
In order to mitigate some of these safety issues associated with high pressure cylinders, on-site electrochemical generation of such gases has been used. Because of difficulties in the on-site synthesis of the gases, a more recent technique of low pressure storage and delivery systems has been to adsorb these gases onto a solid support. These storage and delivery systems are not without their problems. They suffer from poor capacity and delivery limitations, poor thermal conductivity, and so forth.
The following patents and articles are illustrative of low pressure, low flow rate gas storage, and delivery systems.
U.S. Pat. No. 4,744,221 discloses the adsorption of AsH3 onto a zeolite. When desired, at least a portion of the AsH3 is released from the delivery system by heating the zeolite to a temperature of not greater than about 175° C. Because a substantial amount of AsH3 in the container is bound to the zeolite, the effects of an unintended release due to rupture or failure are minimized relative to pressurized containers.
U.S. Pat. No. 5,518,528 discloses storage and delivery systems based on physical sorbents for storing and delivering hydride, halide, and organometallic Group V gaseous compounds at sub-atmospheric pressures. Gas is desorbed by dispensing it to a process or apparatus operating at lower pressure.
U.S. Pat. No. 5,704,965 discloses sorbents for use in storage and delivery systems where the sorbents may be treated, reacted, or functionalized with chemical moieties to facilitate or enhance adsorption or desorption of fluids. Examples include the storage of hydride gases such as arsine on a carbon sorbent.
U.S. Pat. No. 5,993,766 discloses physical sorbents, e.g., zeolites and carbon, for sub-atmospheric storage and dispensing of fluids in which the sorbent can be chemically modified to affect its interaction with selected fluids. For example, a sorbent material may be functionalized with a Lewis basic amine group to enhance its sorbtive affinity for B2H6 (sorbed as BH3).